What is vena contracta in open-channel flow and how does it affect flow measurement across an orifice?

• A. The upstream cross-section immediately before the orifice
• B. The narrowest flow passage downstream of an orifice; causes additional minor losses and effective area is less than the orifice area
• C. The widest part of the jet downstream of the orifice
• D. The area of maximum velocity upstream

Answer: (B)

Vena contracta is the narrowest portion of the jet that forms just downstream of a restriction like an orifice. As the flow squeezes through the opening, the streamlines contract and the cross-sectional area of the jet becomes smaller than the physical orifice area. Because the opening is effectively smaller here, the velocity is higher in this region, and there are contraction-related losses to the flow.

This reduced, downstream cross-section sets the actual discharge through the orifice. The flow is described using a contraction (or discharge) coefficient, which is less than one, to account for the fact that the effective area carrying the flow is smaller than the opening. Downstream, the jet then expands to fill the channel, but the vena contracta determines the initial strength and losses of the jet. This is why the correct description emphasizes the narrowest flow passage just downstream of the orifice and the fact that the effective area is smaller than the orifice area, with associated minor losses.